1. Field of the Invention
The invention relates generally to spectrophotometric methods of measuring trace quantities of metals in jet fuel.
2. Description of the Prior Art
It has been well established that trace quantities of certain metals will promote autoxidation of organic compounds in the presence of oxygen. While it has been observed that iron, zinc, and lead can promote the oxidative degradation of hydrocarbon fuels, copper and its compounds have been shown to be one of the most active and available oxidation initiator instability promoters. Trace levels of dissolved copper (typically 25 μg/L or less) can greatly accelerate the rate and extent of autoxidation in fuels. It is generally believed that catalytic metals or surfaces such as copper initiate free-radical autoxidation through the catalytic formation of hydroperoxy radicals.

In addition, copper can catalyze the decomposition of hydroperoxides to generate additional free radicals through the following mechanismsROOH+Cu(I)→RO.+Cu(II)+OH−ROOH+Cu(II)→ROO.+Cu(I)+H+
Because dissolved copper can operate as both a reducing agent (Cu(I)) and an oxidizing agent (Cu(II)), very low concentrations of copper can cause the rapid decomposition of large amounts of hydroperoxides.
Despite the fact that the copper sweetening process has been almost entirely replaced by processes such as Merox treating and hydrotreating that do not introduce copper contamination, copper can still be acquired by jet fuel from contact with copper-bearing alloys in fuel handling processes, including pipes, brass fittings, bearings, and most significantly, Navy shipboard fuel handling systems which are comprised largely of Cu/Ni admiralty metal. Unfortunately, these low levels of dissolved copper in jet fuel promote oxidation degradation that often leads to the formation of gums and sediments in aircraft fuel systems, thereby impeding engine performance, and exerting a significant impact on maintenance costs. In gas-drive fuel Coker tests, as little as 15 to 25 μg/L of added copper or iron, and 100 to 250 μg/L of added zinc or lead were shown to have deleterious effects on JP-7 thermal stability. There is also evidence that dissolved copper can promote certain prerequisite chemical reactions in jet fuel during storage, which enhance the autoxidation process and result in further thermal degradation.
In a shipboard JP-5 fuel survey conducted by Southwest Technical Institute, out of over 200 samples, the copper ranged between 0 and 838 μg/L, with approximately 10% of the samples containing more than 50 μg/L. The majority of the fleet fuel samples tested were found to fail the standard JFTOT test for thermal stability when they contained 50 μg/L of dissolved copper, and several samples failed with 25 to 50 μg/L copper. Some fuels failed the JFTOT test with as little as 15 μg/L copper, although it is generally agreed that levels above 25 μg/L should be avoided.
The determination of copper in fuel to this level is nontrivial. Graphite furnace-atomic absorption spectroscopy analysis is currently the only method available to reliably quantify copper in fuel at these low levels. Thus, there is no field method currently available to detect low threshold levels of dissolved copper in jet fuel. While there are numerous metallochromic spectrophotometric methods for quantitating copper levels in aqueous media, there have been no reports to date dealing with the direct application of these dyes to fuels.